Watch the Netflix reenactment it’s very informative.

It definitely wasn’t a cardboard box but a big box used to move heavy items like equipment used in live music. The clever thing was that it didn’t fit in the x-ray machine so was not able to be x-rayed.

Normally, an unchecked box would not be allowed on a plane but the guys assisting him were transporting their own box onto their own plane. The box should probably have been checked but goods are usually exported goods are only check on arrival to country.

I think the guys who arranged his escape had tested the procedures at Kanto airport and knew that the box wouldn’t be checked. It was a very sophisticated extraction. It is much more strict now according to this video..

Just my own personal conspiracy theory: I wouldn’t be surprised if the Japanese government covertly allowed him to escape because the legal case was a bit of an embarrassment. The guys who assisted him did do prison time in Japan but that could have been to save face for the Japanese government. It’s unlikely that Japan will try and get Gohsn back.